1. Field of the Invention
The present invention relates to a microwave isolator, and more particularly to an apparatus using radio frequency (RF) components to provide electrical isolation between two sections of a circuit or between two independent circuits.
2. Description of the Related Art
Many electrical applications require that signals be transmitted between two electrical circuits, or between two sections thereof, which require electrical isolation from each other. In many cases the circuits may have large voltage differences between them. One such application requiring isolation arises in RS-232 communication interfaces. The RS-232 interface may require the coupling of two circuits, each being poorly grounded and/or subject to noisy transients. Another such application arises in motor control circuits, where the high side drivers of each phase float between ground and the power supply while the control circuitry remains near ground.
Electrical isolation may be accomplished using relays or transformers. However, these devices result in slow responses, are not compact, and are relatively expensive, making them impractical for many applications. High voltage transistors may also be used to isolate circuits, but high voltage transistors are difficult to integrate with low voltage transistors on a single IC and are otherwise prone to the same disadvantages.
Many of the above disadvantages have been overcome in the prior art by using opto-isolators. These devices utilize optical coupling, rather than the electrical coupling used in transformers, relays and transistors, to link the two electrical circuits. Opto-isolators use a light source located in the input circuit, commonly a light emitting diode (LED), optically coupled to a photodetector located in the output circuit, to couple the two circuits. Current flowing in the input circuit causes the LED to emit light, with some of the light being received by the photodetector, thereby causing electrical current to flow in the output circuit.
Opto-isolators are typically discrete devices, that is, the light source and photodetector are manufactured separately and individually positioned in an optical cavity. The light source and the photodetector are typically formed from different materials. The LEDs are fabricated from Group III-V compounds, that is, a combination of a Group III element and a Group V element as arranged on the periodic table, in order to properly release energy in the infrared wavelength when excited. Some examples of Group III-V compounds commonly used are Gallium arsenide (GaAs) and Galium phosphide (GaP). The photodetectors are commonly formed from Silicon (Si). Considerable care must be exercised in positioning the LED and the photodetector with respect to each other in the cavity, to obtain efficient light coupling. In addition, the material used to form the cavity is often crucial in maximizing light coupling.
While opto-isolators have offered a compact, faster, less expensive, and more practical solution to electrical isolation needs over relays, transformers, and high voltage transistors, they also suffer some disadvantages. For instance, their operating temperature range is typically limited to a maximum of 100xc2x0 C., with the LED being the limiting factor. Since many applications require higher temperature devices, the opto-isolator is not a viable solution. For example, motor control applications typically may require components to withstand temperatures of 160xc2x0 C. Other industrial type applications require similar operating temperatures.
Another disadvantage of the opto-isolator lies in the compounds required to fabricate the LED. Since the LED cannot be formed of silicon (Si), and is instead formed using a Group III-V compound, it becomes impractical to integrate the opto-isolator together with other digital or analog circuitry on a single silicon integrated circuit, in light of the specific requirements regarding the LED material and the optical cavity. Therefore, the opto-isolator requires an additional dedicated integrated circuit chip (IC), separating it from the digital or analog circuitry which it serves to isolate.
Therefore, a need exists for a discrete isolator circuit capable of operating at higher temperatures than the opto-isolators of the prior art and which can be formed entirely of silicon, thereby allowing integration together with other digital or analog circuitry on a single silicon integrated circuit.
It is therefore an object of the present invention to provide a discrete isolator circuit capable of operating at higher temperatures than the opto-isolators of the prior art.
It is another object of the present invention to provide a discrete isolator circuit which can be formed entirely of silicon, thereby allowing integration together with other digital or analog circuitry on a single silicon integrated circuit.
It is still another object of the present invention to provide a discrete isolator circuit which can be formed entirely of silicon, thereby allowing integration together with other digital or analog circuitry and allowing partitioning of the circuitry into separate sections to provide fault tolerance capabilities in a single silicon integrated circuit.
To achieve the above objects, a microwave isolator in accordance with the present invention is provided which comprises a driving circuit and a receiving circuit, wherein the driving circuit receives an input signal from a first isolated circuit and generates a corresponding radio frequency (RF) signal and the receiving circuit detects and decodes the corresponding RF signal and provides a corresponding output signal to a second isolated circuit.
The driving circuit preferably contains an oscillator for generating the corresponding RF signal based on the input signal from the first isolated circuit, a transmitting antenna for transmitting the corresponding RF signal, and a microwave switch interposed between the oscillator and the transmitting antenna for switching the RF signal to the antenna.
The receiving circuit preferably contains a receiving antenna to receive the corresponding RF signal and circuitry for detecting and decoding the received corresponding RF signal and providing the corresponding output signal to the second isolated circuit.
All the above components are fabricated of Silicon (Si), preferably Silicon-on-insulator (SOI), thereby providing an isolator on a single substrate and overcoming the disadvantages of the prior art. Accordingly, complete integration with other digital and analog circuitry on a single silicon circuit is realized. In addition, fault tolerant design among circuit partitions is obtainable. Finally, the higher operating temperatures of Si, especially SOI, can be advantageously obtained, making the microwave isolator ideal for high temperature applications such as motor control.